The objectives of this project include the characterization of descending neural modulation of responses to noxious stimuli. These studies allow inferences about what activity of trigeminothalamic and spinothalamic neurons is necessary and/or sufficient for the perception of painful stimuli as well as providing behavioral data on the relative effects of narcotics on sensory and affective dimensions of pain responsivity. Experiments include: (1) Evaluation of the efficacy of morphine-produced analgesia in rats before and after lesions of spinal cord tracts including descending components of the dorsolateral funiculus; (2) Behavioral assessment in monkeys of the analgesic consequences, as measured by inhibition of flexion reflexes, of electrical stimulation of or narcotic microinjection into midbrain structures; (3) Studies of the effects of electrical stimulation or narcotic microinjection on responses of neurons in trigeminal nucleus caudalis responding to noxious and innocuous somatosensory stimuli; and (4) Studies of the effects of a narcotic and a minor tranquilizer on detection and discrimination of thermal stimuli by monkeys. BIBLIOGRAPHIC REFERENCES: Hayes, R.L., Newlon, P.G., Rosecrans, J.A. and Mayer, D.J.: Reduction of stimulation-produced analgesia by lysergic acid diethylamide, a depressor of serotonergic neural activity. Brain Research 122: 367-372, 1977. Hayes, R.L., Price, D.D. and Dubner, R.: Naloxone antagonism as evidence for narcotic mechanisms. Science 196: 600, 1977.